STAT3 is a transcription factor constitutively activated in 70 % of solid tumors, where it is related to cancer cell proliferation and survival and cancer-associated inflammatory responses. Conversely, inactivating mutations of STAT3 cause hyper IgE syndrome and turn patients more prone to develop tumours. In order for STAT3 to activate the expression of cancer- and inflammation-related genes, it has to form pairs with other STAT3 molecules and move from the cytoplasm to the nucleus or the mitochondria. We have developed a system to visualize these phenomena in living cells.